This invention relates to an improved process for maintaining the activity of phosphonium catalysts during the advancement reaction of an epoxy resin with a polyhydric phenol or thiophenol.
Epoxy resins have long been produced by the reaction of a vicinal epoxide with a compound bearing phenolic hydroxyls in the presence of a catalyst in a so-called advancement reaction. A variety of ctalysts have been reported in the art, including tertiary amines, quaternary ammonium halides, phosphonium halides, phosphonium carboxylates and the like. Processes for using these catalysts, as well as stoichiometric reagents, have been detailed in U.S. Pat. Nos. 2,216,099; 2,633,458; 2,658,855; 3,377,406; 3,477,990; 3,547,881; 3,547,885; 3,569,374; 3,694,407; 3,738,862; 3,948,855; 4,048,141 and 4,177,216. Canadian Pat. No. 893,191, German Pat. DT Nos. 2,206,218 and 2,335,199, the texts Handbook of Epoxy Resins by H. Lee and K. Neville, McGraw-Hill (1967) and Epoxy Resins Chemistry and Technology, edited by C. A. May and Y. Tanaka, Marcel Decker, Inc. (1973) are also of interest.
Tetrahydrocarbyl phosphonium halide, carboxylate, phenate and bisphenate salts have earned favor as catalysts for producing epoxy resins by the reaction of diglycidyl ethers of polyhydric phenols and epihalohydrins. The resulting epoxy resins are generally relatively linear and low in color, when resins having moderate (less than about 30,000) weight average molecular weights are prepared. However, it has now been found that the phosphonium salt, depending on the identity of the salt, is more or less rapidly deactivated as the advancement of the epoxy resin progresses in advancement reactions known in the art. Accordingly, greater quantities of the phosphonium salt than would otherwise be necessary must be employed. Moreover, catalyst deactivation may proceed via reaction with vicinal epoxide end groups, resulting in a resin product having an epoxide content significantly lower than the theoretical value.
It would therefore be advantageous to develop a method of preventing deactivation of the tetrahydrocarbyl phosphonium salt during the epoxy advancement reaction. Also it would be desirable to produce resins in which the advancement catalyst is deactivated without reacting with the epoxy resin product. The subject invention affords these and other advantages.